Version 3.5.1 (latest) of popular blogging engine WordPress suffers from remote denial of service vulnerability. The bug exists in encryption module (class-phpass.php). The exploitation of this vulnerability is possible only when at least one post is protected by a password.

Time frames:
31.05.2013 WordPress security team has been informed about the vulnerability (no response).
07.06.2013 The vulnerability has been released to the public.

More information (including proof of concept):
https://vndh.net/note:wordpress-351-denial-service

A way out (before official WordPress update) to secure existing installations is to apply the following patch:

--- wp-includes/class-phpass.php
+++ wp-includes/class-phpass.php
@@ -120,7 +120,7 @@
       return $output;
 
     $count_log2 = strpos($this->itoa64, $setting[3]);
-    if ($count_log2 < 7 || $count_log2 > 30)
+    if ($count_log2 < 7 || $count_log2 > 13)
       return $output;
 
     $count = 1 << $count_log2;